1. Field of the Invention
An aspect of the present invention may relate to at least an enlargement optical system.
2. Description of the Related Art
An image displaying apparatus is known which may be capable of displaying an enlarged image even if placement thereof is conducted nearer to a screen than a conventional one. Such an image displaying apparatus is called a “short distance projector”. An object(s) of a short distance projector is/are as follows. There are provided, first, to avoid glare provided by projection light entering the eyes of a presenter (an expositor or speaker, etc.) standing near a screen, and second, to prevent an audience listening to an explanation of a presenter from being affected by exhaust air or noise from a projector.
Plural types of a projection optical system included in a short distance projector have been known. For example, there are provided a type being capable of extending an angle of view of a conventional (coaxial or rotationally symmetric) projection optical system to reduce a distance thereof from a screen surface, and a type using a curved mirror, etc. It may be possible for a type extending an angle of view of a conventional projection optical system to achieve a short distance projection as an extension of a conventional technique. However, it may be necessary to provide a lens with a large outer diameter near a screen, and hence, a projector may be entirely large.
On the other hand, it may be possible for a type using a curved mirror to provide a compact projection optical system and to achieve projection at a short distance. An example of a projector including a projection optical system using a curved mirror is described in Japanese Patent No. 4329863 or Japanese Patent No. 3727543. For a projection optical system included in a projector in Japanese Patent No. 4329863, a concave mirror is arranged behind a lens optical system. For a projection optical system included in a projector described in Japanese Patent No. 3727543, a convex mirror is arranged behind a lens optical system. In any thereof, it may be possible to conduct setting by merely arranging a lens(es) and a mirror(s) in order, and hence, it may be possible to increase a precision of arrangement of parts. However, it may be necessary to provide a long distance between a lens optical system and a mirror, and hence, a large projection optical system may be provided.
An invention in which it may be possible to reduce a distance between a lens and a mirror, is described in, for example, Japanese Patent Application Publication No. 2009-157223 or Japanese Patent Application Publication No. 2009-145672. Inventions described in Japanese Patent Application Publication No. 2009-157223 and Japanese Patent Application Publication No. 2009-145672 are to arrange a folding mirror to fold a long distance between a lens optical system and a mirror wherein miniaturization of an optical system is intended.
An invention described in Japanese Patent Application Publication No 2009-157223 is to arrange a concave mirror and a convex mirror in order next to a lens optical system wherein miniaturization thereof is intended. Furthermore, an invention described in Japanese Patent Application Publication No. 2009-145672 is to arrange a plane mirror behind a concave mirror wherein miniaturization thereof is intended.
However, a distance from an image displaying element to a curved mirror is long in an optical system described in any of Japanese Patent Application Publication No. 2009-157223 and Japanese Patent Application Publication No. 2009-145672. Accordingly, a length of an optical system body may interfere in reducing a distance from a screen to a projector body further than a conventional one.
In order to solve such a constraint for “a size of an optical system itself”, an invention described in Japanese Patent No. 4210314 is provided. A projection optical system in which a screen surface and a display surface of an image displaying element are perpendicular to each other is described in Japanese Patent No. 4210314. As such a vertical type is adopted, a length of a projection optical system itself may not provide an interference even if a distance between a screen and a projector body is reduced.
As a vertical type projection optical system such as a projection optical system described in Japanese Patent No. 4210314 is used, it may be necessary to increase a divergence of light incident on a mirror optical system from a lens optical system to project a large format image at an ultra-short throw.
However, as a divergence thereof is increased, a refraction angle of a light ray may be large, and hence, an influence on a quality of a projected image due to a positional displacement of a lens optical system or mirror optical system may be large which may cause degradation of an image quality. In particular, as a position of a lens optical system or a second mirror (a mirror closest to a screen on an optical path) is displaced, a field curvature may vary greatly to provide a factor of a drastic degradation of an image quality.